An Eco-friendly Microorganism Method To Activate Biomass for

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Batteries and Energy Storage

An eco-friendly microorganism method to activate biomass for cathode materials for high performance lithium-sulfur batteries Li Xia, Yibei Zhou, Juan Ren, Huali Wu, Dunmin Lin, Fengyu Xie, Wenjing Jie, Kwok Ho Lam, Chenggang Xu, and Qiaoji Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01453 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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An eco-friendly microorganism method to activate biomass for cathode materials for high performance lithium-sulfur batteries Li Xia1, Yibei Zhou1, Juan Ren1, Huali Wu1, Dunmin Lin1,* , Fengyu Xie1, Wenjing Jie1, Kwok Ho Lam2, Chenggang Xu1, Qiaoji Zheng1,* 1

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China

2

Department of Electrical Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong

Abstract Biomass-based carbon has attracted considerable attention as host materials of active sulfur in lithium-sulfur batteries, while chemical activators of H3PO4, KOH or ZnCl2 are essential to construct the porous structure of the materials. Inspired by traditional Chinese steamed buns, herein a unique porous microcellular carbon composed of cross-linked nanopores has been synthesized by an eco-friendly biological fermentation using banana peel as a carbon precursor. The specially hierarchical carbon framework obtained under the aerobic respiration and anaerobic breathing of biological yeast during fermenting, and simultaneously the inherent doping of N (3.28 at%), produce a promising carbon host material to stabilize the structure of electrodes and restrict the dissolution of polysulfides during charging and discharging. The

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amount of biological yeast has an important influence on the microstructure of the biomass carbons and the correlated electrochemical properties of carbon/sulfur electrodes. The optimal amount of biological yeast is 3.0 wt%, where the carbon/sulfur composite electrode possesses the sulfur loading of 74.34 wt% and achieves a large initial reversible capacity of 1174 mAh g-1 at 0.1 C and a high capacity hold of 58.35% after 100 cycles. Our study provides a novel eco-friendly strategy to fabricate the interconnected hierarchical porous carbon framework from living wastes for various energy storage and conversion applications, including lithium-ion battery, supercapacitor, etc. Keywords: Energy storage, Biomass carbon, Nitrogen doping, Lithium-sulfur battery Introduction Rechargeable lithium-ion batteries (LIBs) have been extensively applied as power sources for various electronic devices and electric vehicles[1]. Nevertheless, traditional cathodes of LIBs like LiFePO4 and LiMn2O4 with inherently low theoretical capacities of 170 mAh g-1 and 148 mAh g-1 are difficult to satisfy the increasing need for energy density

[2-3]

. Nowadays, lithium-sulfur

(Li-S) batteries are attracting increasing interest as one of the most prospective candidates for energy storage devices for its outstanding theoretical energy density (2600 Wh Kg-1) and specific capacity (1675 mAh g-1)[4-8]. In addition, sulfur, serving as earth-abundant element, is non-toxic, inexpensive and environmental friendly[9]. Nevertheless, the practical commercialization of Li-S batteries are still limited by a number of challenges[10]: (1) insulating characteristics of sulfur and discharge

end

products

(Li2S/Li2S2);

(2)

large

volume

expansion

(~80%)

during

charging/discharging; and (3) dissolution of long-chain lithium polysulfides intermediates in electrolyte and their shuttling between electrodes[11-13]. Many investigations have been carried out to overcome the aforementioned drawbacks.


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Among them, the encapsulation of active sulfur into nano-structured porous carbon matrix is an excellent approach to improve the electronic conductivity of insulated sulfur, immobilize soluble long-chain

polysulfides,

and

buffer

the

huge

volume

change

before

and

after

charging/discharging[14-15]. Tang et al.[16] proposed a hierarchical porous graphene, which enhanced the conductivity of the active material. Hu et al.[17] prepared a novel carbon nanotube that achieved the encapsulation of high content sulfur and limited the dissolution of polysulfides. Xie and co-workers[18] synthesized a meso/microporous carbon material, which effectively buffered the volume expansion during charging/discharging. Yu et al.

[19]

reported hierarchical

structure carbon materials derived from hair, which possessed enhanced electrochemical properties. All these porous carbon materials with high conductivity and hierarchical porous structure can improve sulfur utilization and increase electrochemical properties of Li-S batteries because of the supply of fast pathways for e-, Li+ and sulfur, and large spaces for volume expansion[20].

Among

plants[21],dopamine

[22]

porous

carbons,

and wheat straw

[23]

biomass-derived

materials

such

as

reed

have received considerable attention for energy

storage materials in recent years because of their low price, easy availability and specific microstructure

[24]

. For biomass carbon materials, chemical activators, also named as

pore-forming agents, are essential to create pores at micro-, meso- and macro-pore scales[25]. The frequently used activators include KOH, ZnCl2 and H3PO4. For instance, Yuan et al.[26] synthesized porous carbon material by thermal carbonization of corn stalks and subsequent activation of KOH, and the material delivered a capacity of 965 mAh g-1 at 0.2 C; Cui et al.[27] prepared a hierarchical porous materials by activating cotton with ZnCl2, delivering a capacity of 850 mAh g-1 at 1 C; and Celia et al[28]. designed a disordered carbon derived from cherry pits after activated with H3PO4, exhibiting a capacity of 1148 mAh g-1 at 0.05 C. However, it should


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be noted that chemical agents of KOH, ZnCl2 or H3PO4, as activators, are not inherently eco-friendly and economic. Furthermore, the introduction of various chemical activators may lead to a reduction in the volumetric energy density due to the formation of some non-active sulfur impurities[29]. In general, porous carbon materials are non-polar[30] and thus cannot generate the strong bonding interaction to inherently polar polysulfides, which results in the polysulfides separating from the carbon matrix and consequently serious capacity decay in long term cycling[31]. It has been proved that the doping of heteroatoms such as nitrogen[32], phosphorus[33] and sulfur[34] can induce the bonds between heteroatoms and porous carbons, and thus effectively anchor polysulfides, which improves electronic conductivity and increases reaction active sites, consequently enhancing the electrochemical properties of the materials[35-37]. Especially, the doping of nitrogen can significantly change the charge state of neighboring carbon atoms and induce strong chemical adsorption for polysulfides, leading a considerable improvement in electrochemical property of the materials[38-40]. Generally, NH3 or ammonium salt are used as an extrinsic nitrogen resource to be doped into the materials, which usually induces inhomogeneous doping of heteroatoms in carbon matrix [41-43]. Obviously, compared with the extrinsic doping of heteroatoms, inherent doping of heteroatoms is more conducive to the electrochemical property of Li-S batteries[44]. Banana, one of the most popular fruits, are produced and consumed in large quantities every year in the world. It is noted that nearly 1.06 million tons of banana peels are discarded as living wastes every year all over the world[45]. Banana peels contain abundant carbohydrate and protein, and thus may be a promising carbon precursor. In this work, biological fermentation inspired by traditional Chinese steamed buns, as a facile and eco-friendly approach, was first used to prepare the hierarchical porous carbon using waste banana peels as a precursor


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with no chemical activator. Compared with ordinary methods (i.e., chemical activators of KOH, ZnCl2 or H3PO4) for the preparation of biomass porous carbons[46], biological fermentation avoids the utilization of chemical activators, which should be much eco-friendly, economic and simpler. The as-synthesized carbon materials exhibit the hierarchical porous structure and possess with the inherent doping of N, delivering a prominent capacity of 1174 mAh g-1 at 0.1 C with the high sulfur loading of 74.34 wt%. Experimental Synthesis of carbon materials Waste banana peels were utilized as a carbon precursor to synthesize nitrogen-doped hierarchical porous carbon materials using a combined method of biological fermentation and subsequent carbonization. Firstly, banana peels were cut into small pieces, and then smashed by a grinding machine. After that, yeast was mixed with grated banana peels (the mass ratio of yeast to banana peel is 0.0 wt%, 1.0 wt%, 3.0 wt%, 5.0 wt% and 7.0 wt%, respectively) and heated in a water bath at 40 oC for 15 h. Finally, the yeast-fermented banana peel was carbonized at 500 oC for 2 h in N2 (heating rate: 5 oCmin-1). The obtained samples were washed repeatedly with alcohol and ultra pure water until pH is close to 7.0, and then dried in a vacuum oven at 150 oC for 12 h. The as-synthesized carbon materials with the mass ratio of yeast to banana peel of 0.0 wt%, 1.0 wt%, 3.0 wt%, 5.0 wt% and 7.0 wt% were referred as FBPC-0.0 wt%, FBPC-1.0 wt%, FBPC-3.0 wt%, FBPC-5.0 wt% and FBPC-7.0 wt%, respectively. Fabrication of FBPC/S composites The FBPC/S materials were synthesized through a typical stem-melting way on account of the capillary force theory[25]. A two-step process was employed to encapsulate sulfur into the prepared porous carbon substrates: (1) the FBPC was put into the smallest crucible that was


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covered by several alundum crucibles with increasing diameters, in which certain amount of sulfur was shelved between the crucibles. The whole setup was heated at 115 oC for 24 h to develop a sulfur atmosphere; and (2) the resulting porous carbon material was mixed with sulfur in a weight ratio of 3:7, and then the obtained material was heated at 155 oC for 24 h. The composites of biomass carbon materials and sulfur were denoted as FBPC/S-0.0 wt%, FBPC/S-1.0 wt%, FBPC/S-3.0 wt%, FBPC/S-5.0 wt%, FBPC/S-7.0 wt%, respectively. Material characterizations X-ray diffraction (XRD, Smart Lab, Rigaku, Japan), was utilized to examine the crystal structure of the materials. Raman spectroscopy (Renishaw 2000，UK) was performed at 250-4000 cm-1. The microstructure and morphology of the material were observed by scanning electron microscopy (FE-SEM, JSM-7500, and Japan) and transmission electron microscopy (FE-TEM, GZF20，USA). The sulfur content of the material was detected by a thermos analyzer (TGA, DSC-TGA, Q500, American TQ Company，USA) under N2 flowing at the heating rate of 10 oC/min. The specific surface area and pore characteristics of the materials were examined at 77 K (BET, Micromeritics ASAP2020，USA) based on N2 adsorption-desorption isotherm principle. X-ray photoelectron spectroscopy (XPS) analysis was conducted at room temperature on a PHI 5000 Versa Probe XPS spectrometer with an Al Kα radiation source (1486.6 eV) (Thermal ESCALAB 250XI，USA). Electrochemical measurements All electrode materials contain 80 wt% FBPC/S material, 10 wt% conductive acetylene black (super P) and 10 wt% binder (PVDF) mixed in N-methyl-2-pyrrolidone (NMP). After stirring for 4 h, the slurry was uniformly coated onto Al foil and dried in a vacuum oven at 60 oC for 24 h. The resulting aluminum foil was cut into the wafer (diameter:14 mm). The sulfur content of each


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electrode is ~1.3 mg cm-2. The coin-type cells (CR-2032) were assembled using the FBPC/S as cathode, Celgard 2400 as separator and lithium foil as anode in an argon-filled glove box (MB-Labstar, Germany). The electrolyte consists of 1 M LiN(CF3SO2)2 (LiTFSI) dissolved in a blended solvent that composes of 1,3-dioxolane (DOL) and dimethoxymethane (DME) (v/v: 1:1). In addition, the 1wt% LiNO3 (Sigma Aldrich), as additive, was also added to the electrolyte. Galvanostatic charging/discharging measurement was performed by a battery measurement system (CT2100A, LAND, China) at current densities of 0.1-1 C (1 C = 1675 mA g-1) between 1.7 and 2.8 V. Moreover, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted via an electrochemical workstation (CHI660E, Shanghai, China). Cycle voltammetry (CV) was measured at a scanning rate of 0.1 mVs-1 between 1.5 V and 3.0 V. The impedance was acquired in the range of 100 kHz to 10 mHz and fitted by ZView software. Results and discussion Fig. 1a displays schematic diagram of synthesis for FBPC materials derived from waste banana peels. Steamed buns are one of the most popular breakfast foods in China. It is well known that steamed buns are rich in pores with an interlinked architecture. Inspired by this, banana peels containing large amounts of carbohydrate and protein were used as precursor for the first time to synthesize the hierarchical porous carbon framework by the fermentation of yeast. As shown in Fig. 1a, the yeast solution was mixed with the smashed banana peel. The mixture was subjected to heat treatment at 40 oC for 15.5 h in water bath and then carbonized at 500 oC for 2 h under N2 atmosphere. The samples were washed repeatedly with alcohol and deionized water until pH =7, and then dried at 150 oC for 12 h. A two-step way was employed to encapsulate sulfur into the prepared porous carbon substrates

[33]

. Fig. 1b exhibits the conceptual scheme of biological

fermentation of banana peel and the formation illustration of FBPC materials. The fermentation


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of banana peels contains a two-step breathing process: (1) carbohydrate in banana peels reacts with the oxygen from air to generate water and carbon dioxide during the first 0.5 h when yeast performs aerobic respiration (Reaction equation (1): C6H12O6 + 6O2 = 6CO2 + 6H2O). The reaction is vigorous due to sufficient oxygen at early stage and a large amount of carbon dioxide is generated during the reaction process, resulting in plentiful macropores in banana peels; and (2) the resulting banana peels are subjected to anaerobic respiration for ~15 h due to the complete consumption of oxygen. In this step, the carbohydrate is transformed into alcohol and carbon dioxide mildly (Reaction equation (2): C6H12O6 = 2CO2 + 2C2H5OH), resulting in relatively small pores. After the respiration reaction, the yeast-fermented banana peels were carbonized at high temperature to obtain porous materials. The microstructure and morphology of the materials may be readily regulated by varying the mass ratio of yeast to banana peel (0.0~7.0 wt%) during the fermentation process.


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Fig. 1. (a) Synthesis processes of traditional Chinese steamed buns inspired porous carbon materials derived from waste banana peels; (b) conceptual scheme of biological fermentation of banana peel and formation illustration of FBPC materials.


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The SEM micrographs of FBPC-x materials and the corresponding FBPC/S composites are shown in Fig. 2. From Fig. 2a1, the FBPC-0.0 wt% without fermentation exhibits a non-porous plate-like structure. As a result, when sulfur is added into the carbon matrix, a large mass of sulfur aggregates on the surface, exhibiting relatively large particles and suggesting that sulfur cannot be effectively encapsulated into the FBPC-0.0 wt% material due to the non-porous structure of the materials (Fig. 2a2). Unlike the FBPC-0.0 wt%, when the yeast content of x increases to 1.0 wt%, some pores are created in the FBPC-1.0 wt% due to the fermentation of yeast (Fig. 2b1). Compared with the FBPC/S-0.0 wt%, the surface of the FBPC/S-1.0 wt% composite is relatively rough, and only a little amount of pores are presented on the surface of the composite (Fig. 2b2), indicating that sulfur has been well infiltrated into the material. With x increasing to 3.0 wt%, a number of small and large pores coexist and a hierarchical porous structure is formed in the FBPC-3.0 wt% due to sufficient fermentation of yeast (Fig. 2c1). Accordingly, the sulfur can be effectively loaded to the material (Fig. 2c2). However, excess yeast (i.e., x=5.0 wt% and 7.0 wt%) destroys pores and leads to the fragmentization and hardening of the microstructure for the materials with further large content of yeast (Figs. 2d1 and e1), which is attributed to excessive gas release induced by over fermentation. The hierarchical porous structure is not only conducive to loading large amounts of sulfur, but also advantageous to the transport of the electrolyte. In Fig. S1 a-e, the distribution of S, C, N and O in FBPC/S-3.0 wt% composite is performed by SEM, indicating homogeneous distributions of S, C, N and O. As observed in Fig 2f, smashed banana peel is pale yellow in appearance, while yeast-fermented banana peel is brown (Fig. 2g). From Fig. 2, the hierarchical porous framework could be successfully created by introducing yeast into the materials, and the optimal amount of yeast is 3.0 wt%.


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Fig. 2. SEM images of the FBPC-x and FBPC/S-x materials : (a1) FBPC-0.0 wt%; (a2) FBPC/S-0.0 wt%; (b1) FBPC-1.0 wt%; (b2) FBPC/S-1.0 wt%; (c1) FBPC-3.0 wt%; (c2) FBPC/S-3.0 wt%; (d1) FBPC-5.0 wt%; (d2) FBPC/S-5.0 wt%; (e1) FBPC-7.0 wt%; (e2) FBPC/S-7.0 wt%; (f) smashed banana peel; and (g) yeast fermented banana peel.

The TEM images of the FBPC-x are shown in Fig. 3. From Fig. 3a1, there are no obvious pores in FBPC-0.0 wt%, and few white circles can be observed in the material (Fig. 3a2), implying that there are no micro/mesopores in the FBPC-0.0 wt%. These characteristics suggest an almost non-porous microstructure in the FBPC-0.0 wt% without experiencing the fermentation. Unlike the FBPC-0.0 wt%, the FBPC-3.0 wt% exhibits a large number of pores (Fig. 3b1) that are displayed as white spots in Fig. 3b2, indicating that the FBPC-3.0 wt% possesses a hierarchical micro/mesoporous structure. This offers large space for volume expansion during charging/discharging and abundant transmission paths for ions/e-, which is favor of the immersion of electrolytes, the transport of ions/e-, and the anchoring and adsorption


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for sulfur and polysulfides. However, excessive yeast is not conducive to constructing the hierarchical interconnected porous framework in the material. From Fig. 3c1, a few of shallow pores is detected for the FBPC-7.0 wt%. As illustrated in Fig. 3c2, there are few white spots in the FBPC-7.0 wt%, and the color becomes relatively dark. These indicate that excessive yeast leads to low porosity due to the broken pores[37].Together with the SEM images, it can be concluded that after fermented with 3.0 wt% yeast, an interconnected micro/meso/macroporous framework was successfully developed.

Fig. 3. TEM images of the FBPC-x: (a) FBPC-0.0 wt%; (b) FBPC-3.0 wt%; and (c) FBPC-7.0 wt%.

The Raman spectra of the FBPC-x are shown in Fig. 4a. All FBPC-x materials exhibit two distinct peaks, corresponding to D and G bands of FBPC at about 1360 cm-1 and 1585 cm-1, respectively. The D band stands for the disordered carbon structure, while the G band represents the graphite carbon framework. The intensity ratio of ID/IG reflects the defect degree of material[47]. From Fig. 4a, as x increases, the ID/IG ratio of D to G bands increases gradually, giving the values of 0.78, 0.76, 0.87, 0.82 and 0.85 at x = 0.0 wt%, 1.0 wt%, 3.0 wt%, 5.0 wt% and 7.0 wt%, respectively. The FBPC-3.0 wt% possesses the largest ID/IG of 0.87, suggesting the


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highest defect degree and thus the largest number of active sites for immobilizing polysulfides. Fig. 4b displays the XRD patterns of the FBPC-x. For all materials, there is a wide diffraction peak at 20o ~ 30o, suggesting a typical amorphous carbon materials. The other weak peaks in the XRD patterns of the materials are related to some potassium compounds due to the abundance of potassium-containing substances in the banana peels. The XRD patterns of the S and the FBPC/S-x are shown in Fig. 4c. For all FBPC/S-x materials, strong and sharp characteristic peaks are found, corresponding to the orthorhombic crystalline structure of sublimated active materials[48]. The TGA curves of the sulfur and the FBPC/S-x are shown in Fig. 4d. The amount losses of all materials appear between 175 oC and 450 oC, corresponding to the evaporation of S. The content of active materials in the FBPC/S-x composites with x = 0.0 wt%, 1.0 wt%, 3.0 wt%, 5.0 wt% and 7.0 wt% are 70.03 wt%, 70.48 wt%, 74.34 wt%, 73.75 wt% and 71.82 wt%, respectively. The FBPC/S-3.0 wt% composite possesses the highest sulfur content of 74.34 wt%, which should be ascribed to its hierarchical porous structure that can absorb more sulfur in the first step of impregnating sulfur than other carbon matrixes.


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Fig. 4.

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(a) Raman spectra of the FBPC-x; (b) XRD patterns of the FBPC-x; (c) XRD patterns of the sulfur and the FBPC/S-x; and (d) TGA curves of the FBPC/S-x.

The porosities of the FBPC-x were detected by the mercury pressure method. From Fig. 5a, the FBPC-3.0 wt% possesses the highest pore volume of 104%, which is much larger than those of the FBPC-0% and FBPC-7.0 wt% (65% and 24%, respectively). In addition, the FBPC-3.0 wt% exhibits a relatively wide pore size distribution (Fig, 5b), which is in favor of immobilizing the dissolution of long-chain polysulfides and buffering the huge volume change before and after charging/discharging. The specific surface area of the FBPC-3.0 wt% is 112.05 m2g-1. As shown in Fig. 5c, the FBPC-3.0 wt% exhibits a hysteresis loop between 0.3 and 0.99, which is related to the capillary condensation of nitrogen in the micropores and mesopores, demonstrating a micro-mesoporous structure. Fig. 5d shows the pore size distribution of the FBPC-3.0 wt%. It can be observed that some micropores, abundant mesopores and a large number of macropores coexist in the material, which can limit the dissolution of polysulfides and anchor the sulfur in the process of charging and discharging in a long cycle[37]. From Figs 1, 2, 3, and 5, the FBPC-3.0 wt% possesses the highly hierarchical porous structure with large meso/macropore volume (2.0412 cm3 g-1) as well as the distinctive coexistence of micro/meso/macropores, which can contribute to the containment of sulfur inside the nanopores of the materials.


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Fig. 5. (a) Mercury intrusion curves of the FBPC-x in the range of 6 nm to 300 µm; (a) Mercury pore size distribution of the FBPC-x; (c) Nitrogen adsorption and desorption isotherms of the FBPC-3.0 wt% and (d) Pore size distribution of the FBPC-3.0 wt%.

The XPS of the FBPC/S-3.0 wt% is shown in Fig. 6. From Fig. 6a, the carbon is made up by C, O, N and S, giving the O and N contents of 21.28 at% and 3.28 at%, respectively. The high-resolution C1s, N1s and S2p spectra for the FBPC/S-3.0 wt% are shown in Figs. 6b-d, respectively. For the high-resolution C1s spectrum in Fig. 6b, three peaks are observed, corresponding to C-C/C=C at 284.3 eV, C-N/C-O at 284.9 eV and C=O at 287.5 eV, respectively. These suggest the existence of nitrogen/oxygen-functional groups in the material for the enhancement of the binding ability of lithium polysulfides. The high-resolution N1s spectrum in Fig. 6c may be decomposed into four peaks, corresponding to pyridinic N at 398.3 eV, pyrrolic N at 399.7 eV, graphitic N at 401.0 eV and oxidized N at 402.0 eV, respectively. The high-resolution N1s spectrum of the FBPC/S-0.0 wt% and FBPC/S-0.7 wt% are shown in Fig. S2. The ratio of pyridinc-N of FBPC/S-0.0 wt%, FBPC/S-3.0 wt% and FBPC/S-7.0 wt% are


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16.8%, 26.14% and 16.39%, respectively. It is reported that a higher amount of the pyridinic-N is incorporated into the porous carbon framework to act as the polar functional groups to interact with the polysulfides (Li2Sn, 4

An Eco-friendly Microorganism Method To Activate Biomass for

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